1. Field of the Invention
The present invention is directed to high performance fiber-reinforced brittle matrix composites such as those having a cementitious matrix and containing matrix interactive reinforcing fibers.
2. Background Art
Fiber-reinforced cementitious composites have been made for many years. However, only relatively recently have the micromechanics of fiber-reinforced brittle matrix composites been understood so as to significantly improve the physical properties of such composites. For example, in published PCT application No. WO 99/58468, moderate strain hardening is achieved in concrete of normal density by incorporation of both 4% by volume of polyvinyl alcohol fibers and a densified matrix. Despite the relatively high volume percentage of fibers, the strain capacity is still only 0.5%. Moreover, concrete containing a relatively high volume percentage of fibers such as that of WO 99/58468 is difficult to mix, particularly on site.
In U.S. Pat. No. 5,993,537, special polypropylene copolymer fibers which fibrillate during admixture to concrete mixes are said to provide enhanced impact resistance and flexural strength. However, no improvement in uniaxial tensile strength nor tensile ductility is disclosed. In several studies by Li and coworkers, i.e., V. C. Li et al., “Interface Tailoring for Strain-Hardening PVA-ECC,” ACI MATERIALS JOURNAL 99(5):463–472; T. Kanda et al., “Multiple Cracking Sequence and Saturation in Fiber-Reinforced Cementitious Composites,” JCI CONCRETE RESEARCH AND TECHNOLOGY 9(2): 19–33 (1998); V. C. Li, “Post-Crack Scaling Relations for Fiber-Reinforced Cementitious Composites,” ASCE J. OF MATERIALS IN CIVIL ENGINEERING 4(1): 41–57 (1992); V. C. Li, “Engineered Cementitious Composites—Tailored Composites Through Micromechanical Modeling,” In N. Banthia, A. Bentur, A. & A. Multi (eds.) Fiber Reinforced Concrete: Present and the Future: 64–97, Montreal:Canadian Society for Civil Engineering (1998); V. C. Li et al., “Steady State and Multiple Cracking of Short Random Fiber Composites,” ASCE J. OF ENGINEERING MECHANICS, Vol. 188, No. 11, pp. 2246–2264; V. C. Li et al., “Matrix Design for Pseudo Strain-Hardening Fiber Reinforced Cementitious Composites,” RILEM J. MATERIALS AND STRUCTURES, 28(183):586–595 (1995); Z. Lin et al., “On Interface Property Characterizations and Performance of Fiber Reinforced Cementitious Composites,” J. CONCRETE SCIENCE AND ENGINEERING, RILEM 1:173 (1999); and H. C. Wu et al., “Stochastic Process of Multiple Cracking in Discontinuous Random Fiber Reinforced Brittle Matrix Composites,” INT'L. J. OF DAMAGE MECHANICS 4(1):83–102 (1995), and also in studies by others, a much greater understanding of the role of fiber reinforcement in cementitious matrices has been developed. For example, it has been found in general, that fibers which exhibit little interaction with the fiber matrix produce little or no increase in ductility, as measured by uniaxial tensile strain capacity. On the other hand, fibers such as untreated polyvinyl alcohol fibers, which exhibit exceptionally strong matrix interactions, also fail to generate ductile behavior. In the case of polypropylene fibers, for example, as a crack in the composite develops, the polypropylene fibers are easily pulled from the matrix, and multiple cracking cannot occur. As a result, the fibers do little to increase strain once a crack has developed. Fibers such as untreated polyvinyl alcohol, on the other hand, resist pullout to the extent that little elongation takes place until the tensile strength of the fiber is reached, resulting in catastrophic failure.
It has also been found, somewhat counter-intuitively, that fiber-reinforced composites with high matrix fracture toughness exhibit lesser ductility than those with lower matrix fracture toughness. Studies support the theory that lower fracture toughness matrices generate an increased number of cracks. The increased number of fractures allows the tensile strain to be distributed across a wider number of cracks. With the proper volume percentage of fibers, generally about 2.5 volume percent or less, an appropriate fiber-to-matrix interaction, and a suitable matrix, tensile strain of from 3 to 5% may be achieved. Such amounts of strain are not ordinarily associated with cementitious products, which are commonly thought of as brittle and unyielding. Moreover, these latter products, which are termed “high performance fiber-reinforced cementitious composites” (HPFRCC) by the art, actually exhibit increasing strength with increasing strain, a type of ductility commonly associated with metals, but not with cementitious construction compositions.
Unfortunately, increasing the number of cracks in fiber-reinforced cementitious composites has required the use of cementitious matrices of lesser strength. When higher strength, particularly higher fracture toughness matrices are employed, cracking tends to be localized and/or unpredictable, in many cases but few cracks developing under stress, and strain hardening is difficult or impossible to achieve.
Even more important in the use of strain hardening cementitious composites is the change in cracking behavior which occurs due to processing variables. Difficulties associated with achieving thorough mixing are increased when reinforcing fibers are employed, and mixing on site exacerbates these problems. Entrainment of air, for example, is more likely when fibers are included in the matrix, and air pockets provide natural flaws where cracks may initiate and propagate. Such natural flaws also develop where incomplete wetting of aggregate occurs. Unfortunately, such flaws are not uniformly distributed, and often tend to concentrate within limited portions of the composite structure, while other portions are substantially free of such natural crack initiation sites. As a result, strain hardening behavior is much reduced or even absent as compared with test specimens of identical composition prepared under more ideal conditions. An example of this type of behavior is illustrated in FIG. 6.
It would be desirable to provide high strength, fiber-reinforced brittle matrix composites which exhibit saturated multiple cracking under tensile stress, this multiple cracking occurring uniformly over the entire composite. It would be further desirable to employ cementitious compositions of higher matrix strength which are also able to exhibit the multiple cracking suitable for strain hardening behavior, and which provide these desirable properties even when mixing is performed on site.